Functional Characterization of BRASSINAZOLE-RESISTANT 1 in Panax Ginseng (PgBZR1) and Brassinosteroid Response during Storage Root Formation

Abstract

Brassinosteroids (BRs) play crucial roles in the physiology and development of plants. In the model plant <i>Arabidopsis</i>, BR signaling is initiated at the level of membrane receptors, BRASSINOSTEROIDS INSENSITIVE 1 (BRI1) and BRI1-ASSOCIATED RECEPTOR KINASE 1 (BAK1) complex, thus activating the transcription factors (TFs) BRASSINAZOLE RESISTANT 1/BRI1-EMS-SUPPRESSOR 1 (BZR1/BES1) to coordinate BR responsive genes. BRASSINOSTEROIDS INSENSITIVE 2 (BIN2), glycogen synthase kinase 3 (GSK3) like-kinase, negatively regulates BZR1/BES1 transcriptional activity through phosphorylation-dependent cytosolic retention and shuttling. However, it is still unknown whether this mechanism is conserved in <i>Panax ginseng</i> C. A. Mayer, a member of the <i>Araliaceae</i> family, which is a shade-tolerant perennial root crop. Despite its pharmacological and agricultural importance, the role of BR signaling in the development of <i>P. ginseng</i> and characterization of BR signaling components are still elusive. In this study, by utilizing the <i>Arabidopsis</i><i>bri1</i> mutant, we found that ectopic expression of the gain of function form of PgBZR1 (<i>Pgbzr1-1D</i>) restores BR deficiency. In detail, ectopic expression of <i>Pgbzr1-1D</i> rescues dwarfism, defects of floral organ development, and hypocotyl elongation of <i>bri1-5</i>, implying the functional conservation of PgBZR1 in <i>P. ginseng</i>. Interestingly, brassinolide (BL) and BRs biosynthesis inhibitor treatment in two-year-old <i>P. ginseng</i> storage root interferes with and promotes, respectively, secondary growth in terms of xylem formation. Altogether, our results provide new insight into the functional conservation and potential diversification of BR signaling and response in <i>P. ginseng</i>.